1. Field of the Invention:
The present invention relates to an Al-Cr alloy vapor-deposited material which will find use in the application area of plated products such as automotive exhaust mufflers, stove heat reflectors, constituents of waste incinerator, and building materials, which need high corrosion resistance because they are used at high temperatures in a highly corrosive environment. More particularly, it is concerned with an Al-Cr alloy vapor-deposited material which has not only outstanding corrosion resistance but also good overall durability in a high-temperature, corrosive environment on account of its ability to withstand the exfoliation of metallic deposit even when heated for a long period of time.
Among the substrate metals for the Al-Cr vapor-deposited material of the present invention are iron, steel, alloy steel, aluminum, aluminum alloy, copper, copper alloy, titanium, titanium alloy, etc. in the form of plate, rod, pipe, corrugated sheet, and L- or H-section. This specification will be concerned mainly with steel sheets which are most typical among the metallic materials listed above.
2. Description of the Prior Art:
Corrosion resistance at high temperatures is the most important characteristic property required for automotive exhaust mufflers, reflectors of kerosene stoves and gas stoves, and components of waste incinerators, which are exposed to a high-temperature, corrosive environment. Stainless steel has been generally used in these application areas on account of its outstanding corrosion resistance at high temperatures. Recently, stainless steel is being replaced by aluminized steel which is comparatively inexpensive. Aluminized steel is greatly improved in corrosion resistance at high temperature over unplated steel on account of the surface protection provided by aluminum having superior oxidation resistance and corrosion resistance.
Unfortunately, aluminized steel has the disadvantage that when it is exposed to a high-temperature environment where chloride ions exist, the passivation film, which is composed mainly of aluminum oxide and is formed on the deposit surface, is destroyed comparatively easily, with the result that pitting takes place and rust composed mainly of Al(OH).sub.3 as a corrosion product occurs on the deposit surface. As the pitting proceeds further, penetration pits occur, causing the substrate steel sheet to be exposed, and the substrate steel sheet is subject to rapid corrosion with red rust. Another disadvantage of aluminized steel is that the aluminum deposit easily dissolves even in a weak acidic environment, losing its effect of surface protection. Therefore, aluminized steel is not so good in corrosion resistance as expected when used for parts which are exposed to a complex corrosive environment.
Studies are being made to overcome these disadvantages of aluminized steel by alloying aluminum with Zn or Mg for improved corrosion resistance. However, alloying is not so effective in improving corrosion resistance for the reasons give below.
(1) The alloying metal is limited in its amount to be added because it adversely affects the formability of aluminum. PA1 (2) The hot dip aluminizing process to form the aluminum deposit or aluminum alloy deposit does not permit the alloying metal to be dissolved in the molten aluminum bath infinitely.
Another disadvantage of the hot dip aluminizing process is that the dipping of steel in a molten aluminum bath, during which the surface temperature of steel reaches about 700.degree. C., forms Fe-Al intermetallic compounds such as Fe.sub.2 Al.sub.5 at the interface between steel and aluminum deposit. Being brittle and poor in formability, these intermetallic compounds cause exfoliation when the aluminized steel undergoes intensive forming such as bending.
The formation of Fe-Al intermetallic compounds at the interface between the deposit layer and the substrate metal at the time of plating may be suppressed to some extent by adding about 9% of Si to the molten aluminum bath. The silicon thus added also provides heat resistance, that is, it prevents Fe-Al intermetallic compounds from occurring when the aluminized steel is used at high temperatures, especially above 400.degree. C. However, the effect of silicon is not necessarily satisfactory.
Attempts are being made to provide aluminized steel sheets with improved heat resistance that can be used at high temperatures above 700.degree. C., by replacing the substrate steel with extremely low carbon Cr-Ti killed steel or Cr-Cu killed steel. Even such special killed steels do not completely prevent the formation of Fe-Al intermetallic compounds at the interface between the deposit layer and the substrate steel sheet so long as the hot-dip plating is employed. Therefore, the resulting aluminized steel sheets are not satisfactory in formability. In addition, the above-mentioned special killed steels are more expensive than ordinary Al killed steels, and this offsets the economical advantage of aluminized steel sheets.
Al-Si alloy-plated steel sheets are slightly inferior in corrosion resistance to pure Al-plated steel sheets; therefore, the former is not considered to be superior in overall properties. A common practice to overcome this disadvantage is to provide the surface of Al-Si alloy-plate steel sheets with a thin protective film by chemical conversion such as chromate treatment. This practice, however, is not satisfactory for the improvement of corrosion resistance, because once the protective film is broken or lost by corrosion, the Al-Si deposit layer with insufficient corrosion resistance is subject to corrosion.
Among other plating methods is electroplating. Electroplating is commonly used for electrolytic galavanized steel sheets; but it cannot perform the electrodeposition of aluminum or aluminum alloy on a steel sheet from an aqueous solution. Although electroplating from a non-aqueous solution is basically possible, it is low in efficiency on account of the very low current density at the time of electrodeposition. In addition, the non-aqueous solution for plating is electrically unstable. For these reasons, electroplating is not suitable for the plating of aluminum or aluminum alloy in industrial use.